Removable film, a substrate with film, a process for forming the removable film and a process for the manufacturing of the circuit board

ABSTRACT

Removable mask films 303 are formed on the both sides of the substrate having the adhesive layer 302 by applying and drying a resin varnish 304 including a ultraviolet-absorbing agent, and fine through holes 306 are formed by using a third harmonics YAG solid-state laser light with a relatively short wavelength not longer than that in the ultraviolet range in such a way that the effects of such a residual strain as the conventional embodiment forming a removable mask film by a laminating process may be decreased as well as the more fine hole drilling compared with conventional embodiment using the carbon dioxide gas laser with a relatively long wavelength may be performed.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a process for the manufacturingof a circuit board which interconnects at least two circuit patterns, aremovable film suitable for a process for the manufacturing of thecircuit board, a substrate with film and a process for forming theremovable film.

[0003] 2. Description of the Related Art

[0004] As electronic equipment, in recent years, downsize and becomedenser starting with information-telecommunication, a circuit board isdeeply required to brought to the multilayer not only in the field ofthe industrial use but also in the field of the consumer use. It isrequired in multilayered circuit board to develop an interconnectionstructure contacting through interstitial via hole between a pluralityof circuit patterns. Further, it is required to develop a structure anda process with a high degree of reliability in a structure ofinterstitial via hole contact and a process for preparing its structure.

[0005] The present applicant proposed a novelly configured circuit boardinterconnecting between layers through a interstitial via hole byconductive paste and a novel process for preparing the circuit board.

[0006] A process for the manufacturing of the circuit board is composedas follows.

[0007] That is, a process for the manufacturing of the circuit board iscomposed, comprising:

[0008] a procedure for pasting removable mask film to porous insulatingsubstrate consisting of a composite material of nonwoven fabric andthermosetting resin and having compressibility and providing a throughhole in the substrate;

[0009] a procedure for filling conductive paste into said through hole;

[0010] a procedure for removing said film from said substrate with filmfilled with conductive paste;

[0011] a procedure for bonding metal foil to a face removed of film ofsaid substrate; and

[0012] a procedure for compressing said substrate bonded with said metalfoil by heating and pressuring.

[0013] In such a process for the manufacturing of the circuit board,carbon dioxide gas laser is generally used in drilling porous insulatingsubstrate including removable mask film for providing through hole.

[0014] However, carbon dioxide gas laser is not easy to focus and hardto reduce the focus spot since a wavelength of laser light is relativelyas long as 10.6 μm. Furthermore, since a wavelength of laser lightexists in a range of infra-red rays, through hole is enlarged due to thethermal effects of laser. Therefore, it is difficult to form throughholes, for example, as fine as the diameter of 50 μm and this poses thebarrier for realizing a circuit board of fine interconnection design.

[0015] Further, in a process for the manufacturing of a circuit boarddescribed above, a removable mask films are pasted to the porousinsulating substrate by thermal laminating process. Further, holedrilling is performed on a porous insulating substrate with removablemask films by utilizing laser processing and the like. Furthermore,conductive paste is filled into the drilled holes and thereafter theremovable the mask film is removed.

[0016] In such a process, it is in a condition that strain developed byheat, pressure and film tension in laminating remains in said substrate.When removable film is removed from said substrate under this condition,said residual strain is released from said film and drilled holes resultin a deviation from the hole positions at drilling. This causes asignificant problem particularly in the case required for narrow holepitch and fine dimensional accuracy, that is, in realizing a circuitboard of fine interconnection design.

SUMMARY OF THE INVENTION

[0017] Therefor, it is an principal object of the present invention toprovide a process for the manufacturing of a circuit board which mayrealize a circuit board of fine interconnection design as well as toprovide a removable film suitable for a process for the manufacturing ofthe circuit board, a substrate with film and a process for forming theremovable film.

[0018] It is another object of the present invention to provide acircuit board which protects a drilled hole from being filled togetherwith scraping and dust generated during drilling in filling conductivepaste into the drilled hole and enables via hole contact of low initialresistance and high interconnection reliability.

[0019] To accomplish objectives described above, the present invention,in short, employs removable mask film absorbing laser light ofwavelength not longer than that in the ultraviolet range. Thus, holedrilling may be performed by using laser light of wavelength not longerthan that in the ultraviolet range in which laser beam is easy to focusmore and fine hole drilling may be performed.

[0020] Further, said substrate is preferably a film substrate havingoptical absorptivity upon laser light of wavelength not longer than thatin the ultraviolet range and adhesion or preferably a porous substratehaving optical absorptivity upon laser light of wavelength not longerthan that in the ultraviolet range and compressibility. Thereby, itbecomes that fine hole drilling may be performed on a substrate andremovable film at one time by using laser light of wavelength not longerthan that in the ultraviolet range.

[0021] Further, a removable film preferably contains aultraviolet-absorbing agent. Thereby, it becomes that a removable filmmay absorb laser light with a wavelength not longer than that in theultraviolet range by addition of a ultraviolet-absorbing agent eventhough a principal material composing a removable film does not have aabsorption characteristic of ultraviolet light.

[0022] Further, a removable film is preferably cellulosic, such as nitrocellulose, acethyl cellulose, cellulose acetate, cellulose propionate,ethyl cellulose and the like. Thereby, it becomes that a removable filmmay be formed by using cellulosic having the good ability of the film tobe formed and the good solubility into the solvent.

[0023] Further, preferably, said removable film is formed by applyingand drying a resin varnish, which exerts removability after drying andhas optical absorptivity upon laser light in a range of wavelength notlonger than that in the ultraviolet range, on the surface of thesubstrate. In conventional embodiments forming removable mask films onsubstrates by thermal laminating process, a residual strain develops inthe substrate due to heat, pressure and film tension in laminating andthe residual strain is thereafter released in removing the films toresult in a dimensional deviation. On the contrary, in the presentprocess using a resin varnish, such a dimensional deviation will beimproved.

[0024] Further, a removable film is preferably polyethylene naphthalate(PEN), polyamide or polyimide.

[0025] Further, said resin varnish preferably contains a solvent noteroding said substrate. Thereby, the removable film will be able to beformed on the substrate without eroding the substrate.

[0026] Further, a plurality of removable films are preferably laminatedand provided on a substrate. Thereby, a superficial removable film maybe removed after hole drilling, and scraping generated in hole drillingand dust deposited during a processing may be removed together with aremovable film removed. Thereby, a drilled hole is protected from beingcontaminated with scraping and dust during filling a conductive elementinto the drilled hole. Therefore, a circuit board having via holecontact of low initial resistance and high interconnection reliabilitymay be obtained. Furthermore, since at least one removable film is lefton the side of the substrate after a superficial removable film isremoved, the removable film functions as a mask film in filling theconductive element and therefore a surface of a insulating resin sheetis not contaminated with the conductive element.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Other and further objects of this invention will become obviousupon an understanding of the illustrative embodiments about to bedescribed or will be indicated in the appended claims, and variousadvantages not referred to herein will occur to one skilled in the artupon employment of the present invention in practice.

[0028]FIG. 1 are sectional views of the steps illustrating a process forthe manufacturing of a circuit board in the first preferred embodimentof the present invention;

[0029]FIG. 2 are sectional views of the steps illustrating a process forthe manufacturing of a circuit board in the second preferred embodimentof the present invention;

[0030]FIG. 3 are sectional views of the steps illustrating a process forthe manufacturing of a circuit board in the third preferred embodimentof the present invention;

[0031]FIG. 4 are sectional views of the steps illustrating a process forthe manufacturing of a circuit board in the fourth preferred embodimentof the present invention;

[0032]FIG. 5 are sectional views of the steps illustrating a process forthe manufacturing of a circuit board in the fifth preferred embodimentof the present invention;

[0033]FIG. 6 is a graph showing measurements of resistance in varying avia hole pitch in a circuit board prepared in the fifth embodiment;

[0034]FIG. 7 are sectional views of the steps illustrating the firsthalf of steps of a process for the manufacturing of a circuit board inthe sixth preferred embodiment of the present invention;

[0035]FIG. 8 are sectional views of the steps illustrating the latterhalf of steps of a process for the manufacturing of a circuit board inthe sixth preferred embodiment of the present invention;

[0036]FIG. 9 is a graph showing optical absorptivity of polyethylenenaphthalate (PEN);

[0037]FIG. 10 is a graph showing optical absorptivity of polyethyleneterephthalate (PET);

[0038]FIG. 11 is a graph showing optical absorptivity of ethyl celluloseadded with a ultraviolet-absorbing agent; and

[0039]FIG. 12 is a graph showing optical absorptivity of a singlesubstance of ethyl cellulose.

DETAILED DESCRIPTION OF THE INVENTION

[0040] In the following are described preferred embodiments of thepresent invention taken in connection with drawings.

[0041] First Preferable Embodiment

[0042]FIG. 1 are sectional views of the steps illustrating a process forthe manufacturing of a circuit board according to the first preferredembodiment of the present invention.

[0043] At first, as shown in FIG. 1A, electrical insulating film 101having the adhesive layers 102 on both sides is prepared as a substrate.Film 101, for example, may be selected from a group of polyimide film,aramid film, poly(p-phenylene benzobisoxazole) film, total aromaticpolyester base liquid crystal polymer, and the like. Adhesive, forexample, such as epoxy base, modified polyimide base, silicon base andthe like may be used as the adhesive layer 102. In any kind of adhesive,adhesive is brought to half-cured condition to ensure the ability ofinterconnection layer 107 to be buried in heating and pressuring asdescribed hereinafter.

[0044] An example of film 101 will be named. For example, “KAPTON” (DuPont-Toray trademark), “UPILEX” (Ube Industries Ltd. trademark),“APICAL” (Kaneka Corporation trademark) are named as film 101 comprisingpolyimide film. With these products, water-absorbing capacity may beselected by modifying the kind of the products and film of lowwater-absorbing power is also available.

[0045] For example, “Aramica” (Asahi Chemical Industry Co., Ltd.trademark), “MICTRON” (Toray Co., Ltd. trademark) and “Technora” (TeijinLtd.) are named as film 101 comprising aramid film. These aramid film ishigher in rigidity and more resistant to stretching compared withpolyimide film.

[0046] For example, “Zylon” (Toyobo Co., Ltd. trademark) is named asfilm 101 comprising poly(p-phenylene benzobisoxazole) film. This filmhas heat-resistance, high elasticity and low water-absorbing power.

[0047] For example, “Vectra” (Poly Plastic trademark) is named as film101 comprising total aromatic polyester base liquid crystal polymer.This film is less in heat-resistance but less in water-absorbing andbetter in a dielectric characteristic.

[0048] In this embodiment, film “UPILEX” in thickness of 12.5 μm as afilm 101 and adhesive of modified polyimide base as an adhesive layer102 are used respectively. Modified polyimide base resin is brought tothe half-cured condition by drying after applying to ensure the abilityof a pattern to be buried. A thickness of the adhesive layer 102 is setto 5 μm respectively on each side.

[0049] Then, as shown in FIG. 1B, the removable mask films 103 areformed on the both sides of the film 101 having the adhesive layers 102.Herein, the removability which a mask film has means a degree ofremovability such that a mask film 103 may be removed from a film 101 asa substrate without fracturing a mask film.

[0050] A film absorbing laser light with wavelength not longer than thatin the ultraviolet range, laser light with wavelength of 400 nm or lessin this embodiment, is used as the mask film 103. There are, forexample, polyethylene naphthalate (PEN), polyamide film, polyimide filmand the like as such a film.

[0051] Or, a film composed with a ultraviolet-absorbing agent added mayalso be used other than said film materials. A publicly knownultraviolet-absorbing agent starting with a class of benzotriazole orbenzophenone may be used as the ultraviolet-absorbing agent.

[0052] There are, for example, 2-(2′-hydroxy-5′-methylphenyl)-benzotriazole, 2-(2′-hydroxy-3′-tert-butyl-5′methylphenyl)-5-chlorobenzotriazole and6-(2-benzotriazole)-4-t-octyl-6′-t-butyl-4′-methyl-2, 2′-methylenebisphenol and the like as the ultraviolet-absorbing agent consisting ofa class of benzotriazole.

[0053] There are 2, 2′-dihydroxy-4, 4′-dimethoxy-benzophenone, 2, 2′, 4,4′-tetrahydroxy-benzophenone and the like as the ultraviolet-absorbingagent consisting of a class of benzophenone.

[0054] And, each mask film 103 is provided with a silicon base releasinglayer on the side bonded to the film 101 as required.

[0055] In this embodiment, a polyethylene naphthalate (PEN) film with athickness of 9 μm is used as mask films 103.

[0056] There are a laminating process and a pressing process as methodfor pasting the mask film 103 to the film 101 having the adhesive layer102. In this embodiment, a laminating process is adopted. Laminating isperformed at a temperature of the order of 130° C. This processingallows a surface of the adhesive layer 102 to be melted a little and themask film 103 to be pasted to the film 101.

[0057] Through holes 104 are in turn formed by laser processing in thefilm 101 having mask films 103 and layers of adhesive 102 respectivelyat both sides of the film as shown in FIG. 1C. In this embodiment, ashort-wavelength laser in which laser beam is easy to focus more isutilized as laser light used for laser processing. Specifically, a thirdharmonics YAG solid-state laser radiating laser light with a wavelengthof 351 nm being not longer than that in the ultraviolet range is used.

[0058] An adhesive layer 102 and a film 101 have a characteristic toabsorb laser light with a wavelength in the ultraviolet range. And, amask film 103 has a characteristic to absorb laser light with awavelength of 400 nm or less. Therefore, through holes 104 as fine asthe diameter of 50 μm may be formed with a high forming precision byforming through holes 104 using a third harmonics YAG solid-state laserlight with a wavelength of 351 nm.

[0059] Then, as shown in FIG. 1D, conductive paste 105 is filled intothe through holes 104. In this embodiment, the conductive paste 105 isfilled by printing the conductive paste directly from on the mask film103 by a screen printing machine. In this time, a resin ingredient inthe conductive paste 105 within the through holes 104 is drawn byevacuating and absorbing through a porous sheet (not shown) such asJapanese paper and the like from the opposite side to the printed faceunder vacuum. The conductive paste 105 is increased in a percentage of aconductive element by this operation and further closely filled.

[0060] When such procedures are performed, the mask film 103 plays rolesas a printing mask and a protector against contamination of the surfaceof the adhesive layer 102.

[0061] Then, as shown in FIG. 1E, the mask films 103 are removed fromthe both sides of the film 101. In this procedure, an effect of aportion of a hole end is not negligible in removing since the throughholes 104 are as fine as a diameter of 50 μm. That is, the conductivepaste 105 within the through holes 104 is rifled more or less togetherwith the mask film 103. And, since a diameter of the through holes 104is fine, a amount of the rifled paste thereof comes to relativelyconsiderable amount.

[0062] In this case, though a shape of the remaining conductive paste105 takes a wide variety of forms, the paste is not rifled below thesurface of the adhesive layer 102. The remaining conductive paste 105 isflush with the adhesive layer 102 even at the worst. A phenomenon thatconductive paste 105 is thus taken away through removing the mask film103 (hereinafter, referred to as “phenomenon of the paste being takenaway”) is brought to the fore from and under 100 μm in a diameter of thethrough hole.

[0063] Further, in FIG. 1E, a change of the dimensions of film 101 was50 to 80 μm relative to a reference distance of 60 mm after the maskfilter 103 was removed.

[0064] Then, as shown in FIG. 1F, an interconnection layer 107 supportedby a supporting substrate 106 and a copper foil 108 are overlaidrespectively on the both sides of the film 101. In overlaying procedure,both of the them are overlaid in such a way the interconnection layer107 is at least located directly above the through hole 104 filled withconductive paste 105. And, both of them overlaid are heated andpressurized. Heating and pressurizing is carried out, for example, byusing vacuum press.

[0065] The adhesive layer 102 is fluidized by these heating andpressurizing, and the interconnection layer 107 is buried into theadhesive layer 102 as shown in FIG. 1G. The conductive paste 105 withinthe through holes 104 is compressed by that the interconnection layer107 is buried into the adhesive layer 102 like this, and therefore aresin ingredient in the conductive paste 105 flows out into the adhesivelayer 102 and a conductive element in the conductive paste 105 isclosely packed. Thus, the interconnection layer 107 and the copper foil108 which are located on the opposite sides of the film 101 areelectrically interconnected through the intermediary of conductive paste105. Hereafter, the adhesive layer 102 and the conductive paste 105 arecured.

[0066] Then, as shown in FIG. 1H, the copper foil 108 is patterned intodesired interconnection form using photolithography. In this embodiment,a foil in 9 μm thickness is utilized for the copper foil 108.

[0067] At the last step, the supporting substrate 106 is removed leavingthe interconnection layer 107 buried into the adhesive layer 102 asshown in FIG. 1I. Thus, a circuit board having the interconnections onthe both sides is completed. In this embodiment, aluminum foil is usedfor the supporting substrate 106 and copper foil (patterned) in 9 μmthickness is utilized for the interconnection layer 107.

[0068] Removing the supporting substrate 106 is performed by dissolvingand removing the supporting substrate 106 by selective etching betweenaluminum and copper foils. Other than selective etching, there is also amethod of removing the supporting substrate 106 by heating withreleasing foam sheet provided between the supporting substrate 106 andthe interconnection layer 107. In this method, the supporting substrate106 is not limited to the aluminum foil, and metal foil and metal plateof materials such as copper, stainless and the like may be utilized.

[0069] Further, in this embodiment, a multilayer interconnection boardmay be formed by treating products prepared by the procedure of FIG. 1Ato FIG. 1H as the interconnection layer 107 supported by the supportingsubstrate 106 in FIG. 1F and by repeating the procedures of FIG. 1F toFIG. 1H.

[0070] In accordance with this embodiment, fine through holes 104, forexample, such as the diameter of 50 μm may be formed because holes aredrilled by using a YAG solid-state laser radiating laser light with ashort wavelength not longer than that in the ultraviolet range andtherefore a high density circuit board having a fine via hole may berealized.

[0071] Second Preferable Embodiment

[0072]FIG. 2 are sectional views of the steps illustrating a process forthe manufacturing of a circuit board in the second preferred embodimentof the present invention.

[0073] At first, as shown in FIG. 2A, the removable mask films 202 areformed on the both sides of the porous insulating substrate the 201having compressibility. The porous insulating substrate 201 is, forexample, prepared as follows. That is, thermosetting resin isimpregnated into nonwoven fabric utilizing organic fiber. Thereby,porous insulating substrate 201 having voids inside and compressibilityis prepared. Then, thermosetting resin impregnated is brought to thehalf-cured condition. And, publicly known heat-resistant fiber, forexample, such as aromatic polyamide fiber, poly(p-phenylenebenzobisoxazole) fiber, polybenzimidazole fiber or the like may be usedas organic fiber. And, for example, epoxy resin, polyimide resin, phenolresin, fluororesin, cyanate ester resin or the like may be used asthermosetting resin.

[0074] In this embodiment, nonwoven fabric impregnated with epoxy resinwhich uses aromatic polyamide fiber is used as the porous insulatingsubstrate 201. And, a thickness of the porous insulating substrate 201is 120 μm.

[0075] A film absorbing laser light with wavelength of 400 nm or less inthe range of wavelength not longer than that in the ultraviolet range isused as mask films 202. There are, for example, polyethylene naphthalate(PEN), polyamide film, polyimide film and the like as such a film.

[0076] Or, a film composed with a ultraviolet-absorbing agent added mayalso be used other than said film materials. The ultraviolet-absorbingagent described in the first embodiment may be used as anultraviolet-absorbing agent. Further, mask film 202 may be provided witha silicon base releasing layer as required.

[0077] In this embodiment, a polyethylene naphthalate (PEN) film with athickness of 9 μm is used for mask films 202. There are a laminatingprocess and a pressing process as method for forming the mask films 202on the both sides of the porous insulating the substrate 201. In thisembodiment, a laminating process is adopted. Laminating is performed ata temperature of the order of 120° C. This processing allows a surfaceof the porous insulating substrate 201 to be melted a little and the themask films 202 to be pasted.

[0078] Through holes 203 are in turn formed in the porous insulatingsubstrate 201 provided with mask films 202 by laser processing as shownin FIG. 2B. In this processing, through holes 203 are formed by using athird harmonics YAG solid-state laser radiating laser light with awavelength of 351 nm as well as the first embodiment described above.

[0079] The porous insulating substrate 201 has a characteristic toabsorb laser light with a wavelength in the ultraviolet range and a maskfilm 202 has also a characteristic to absorb laser light with awavelength of 400 nm or less as described above. Therefore, throughholes 203 may be formed by using a third harmonics YAG solid-state laserradiating laser light with a wavelength of 351 nm. In this embodiment,through holes 203 of the diameter of 100 μm are formed by using a laserlight described above.

[0080] Then, as shown in FIG. 2C, conductive paste 204 is filled intothe through holes 203. In this embodiment, the conductive paste 204 isprinted directly from on the mask film 202 by a screen printing machine.In this time, a resin ingredient in the conductive paste 204 within thethrough holes 203 is drawn by evacuating and absorbing through a poroussheet (not shown) such as Japanese paper and the like from the oppositeside to the printed face under vacuum. The conductive paste 204 isincreased in a percentage of a conductive element by this operation andfurther closely filled.

[0081] When such procedures are performed, the mask film 202 plays rolesas a printing mask and a protector against contamination of the surfaceof the porous insulating substrate 201.

[0082] Then, as shown in FIG. 2D, the mask films 202 are removed fromthe both sides of the porous insulating substrate 201. In thisprocedure, since a diameter of the through holes 203 are 100 μm, aphenomenon of “the paste being taken away” described in the firstembodiment seldom occurs.

[0083] Further in this time, a change of the dimensions of the porousinsulating substrate was 201 was 30 to 50 μm relative to a referencedistance of 60 mm after the mask filter 202 was removed.

[0084] Then, as shown in FIG. 2E, copper foils 205 are overlaidrespectively on the both sides of the porous insulating substrate 201.And, both of them overlaid are heated and pressurized. Heating andpressurizing is carried out, for example, by using vacuum press.

[0085] The conductive paste 204 within the through holes 203 iscompressed and therefore a resin ingredient in the conductive paste 204flows out into the porous insulating substrate 201 by these heating andpressurizing, as shown in FIG. 2F.

[0086] Thus, a conductive element in the conductive paste 204 is closelypacked and the copper foils 205 which are located on the opposite sidesof the porous insulating substrate 201 are electrically interconnectedeach other through the intermediary of conductive paste 204. Hereafter,the porous insulating substrate 201 and the conductive paste 204 arecured.

[0087] Then, as shown in FIG. 2G, the copper foil 205 is patterned intodesired interconnection form using photolithography. In this embodiment,a foil in 18 μm thickness is utilized for the copper foil 205. Thus, acircuit board having the interconnections on the both sides iscompleted.

[0088] Further, in the second embodiment, a multilayer interconnectionboard may be formed by treating the circuit board prepared by theprocedures of FIG. 2A to FIG. 2G as the copper foil on the one side inFIG. 2E and by repeating the procedures of FIG. 2E to FIG. 2G.

[0089] Third Preferable Embodiment

[0090]FIG. 3 are sectional views of the steps illustrating a process forthe manufacturing of a circuit board in the third preferred embodimentof the present invention.

[0091] At first, as shown in FIG. 3A, electrical insulating film 301having the adhesive layers 302 on both sides is prepared. Materialsdescribed in the first embodiment may be used as film 301 and adhesivelayers 302. In this embodiment, film “UPILEX” in thickness of 12.5 μm asa film 301 and adhesive of modified polyimide base as an adhesive layer302 are used respectively.

[0092] The adhesive layer 302 was brought to half-cured condition bydrying after applying to ensure the ability of a pattern to be buried. Athickness of the adhesive layer 302 is set to 5 μm respectively on eachside.

[0093] Then, as shown in FIG. 3B, the removable mask films 303 areformed on the both sides of the film 301. The mask films 303 are formedby applying and drying a resin varnish 304. The resin varnish 304 iscomposed of at least a resin to be a principal component of the maskfilm 303, a ultraviolet-absorbing agent and solvent. Cellulosic, forexample, such as nitrocellulose, acethyl cellulose, cellulose acetate,cellulose propionate, ethyl cellulose and the like may be used for aresin to be a principal component of the mask film 303.

[0094] When the mask film 303 is formed as a principal component ofcellulosic, a ultraviolet-absorbing agent described in the firstembodiment is added in such a way that hole drilling may be performed byusing laser light with short wavelength of 400 nm or less in the rangeof wavelength not longer than that in the ultraviolet range. Further, asfor solvent composing of the resin varnish 304, it is selected such asolvent that dissolves the principal component composing of the maskfilm 303 and the ultraviolet-absorbing agent but does not erode theadhesive layer 302. Transformation of the foregoing material mixed intovarnish may be achieved by using in combination adequately dispersionequipment using a media such as a ball mill, a sand mill or the like,kneading equipment such as a pressure kneader, a planetary mixer or thelike and mixing equipment such as a disper or the like. Theconcentration and the viscosity of the resin varnish 304 may bedetermined corresponding to a application means.

[0095] The method for applying the resin varnish 304 on the film 301 isnot limited, a publicly known method such as electrostatic coating, dipcoating, spray coating, roller coating, doctor blade method, gravuremethod, die coating and screen printing etc may be adopted.

[0096] In this embodiment, the resin varnish 304 is prepared by mixingand dispersing materials having the following compositions.

[0097] Ethyl cellulose (ETHOCEL STD100:Dow Chemical Company)

[0098] 100 parts by weight

[0099] Ultraviolet-absorbing agent (JF-77: Johoku Chemical Co., Ltd.) 1parts by weight

[0100] Toluene 450 parts by weight

[0101] Ethanol 450 parts by weight

[0102] After the adhesive layers 302 are formed on the both sides of thefilm 301, the mask films 303 are formed by applying a resin varnish 304on the film 304. The resin varnish 304 is applied by die coating using adie nozzle 305. A thickness of the mask films 303 is adjusted to 10 μmafter it is formed in such a way and then dried. Drying is performed ata temperature required for releasing the solvent e.g. the order of 100°C. Futher, the ultraviolet-absorbing agent in this embodiment (JF-77:Johoku Chemical Co., Ltd.) is composed of 2-(2′-hydroxy-5′-methylphenyl)-benzotriazole to be a principal component.

[0103] Through holes 306 are in turn formed in the film 301 having maskfilms 303 and layers of adhesive 302 respectively on the both sides ofthe film as shown in FIG. 3C. Through holes 306 are formed by laserprocessing.

[0104] In this embodiment, a third harmonics YAG solid-state laserradiating laser light with a wavelength of 351 nm being not longer thanthat in the ultraviolet range is used to perform a fine hole drilling aswell as the first embodiment described above. An adhesive layer 302 anda film 301 have a characteristic to absorb laser light with a wavelengthin the ultraviolet range and a mask film 303 has also a characteristicto absorb laser light with a wavelength of 400 nm or less as describedabove. Therefore, through holes 306 are formed by using a thirdharmonics YAG solid-state laser radiating laser light with a wavelengthof 351 nm. In this embodiment, through holes 306 of the diameter of 50μm are formed by using such a technique.

[0105] Then, as shown in FIG. 3D, conductive paste 307 is filled intothe through holes 306. In this embodiment, the conductive paste 307 isfilled by printing the conductive paste 307 from on the mask film 303 bya screen printing machine. In this time, a resin ingredient in theconductive paste 307 within the through holes 306 is drawn by evacuatingand absorbing through a porous sheet (not shown) such as Japanese paperand the like from the opposite side to the printed face under vacuum.The conductive paste 307 is increased in a percentage of a conductiveelement by this operation and further closely filled.

[0106] When such procedures are performed, the mask film 303 plays rolesas a printing mask and a protector against contamination of the surfaceof the adhesive layer 302.

[0107] Then, as shown in FIG. 3E, the mask films 303 are removed fromthe both sides of the film 301. In this procedure, since the throughholes 306 are as fine as a diameter of 50 μm, an effect of a portion ofa hole end is not negligible and a phenomenon of “the paste being takenaway” described in the first embodiment occurs.

[0108] Further, in FIG. 3E, a change of the dimensions of film 301 waswithin 10 μm relative to a reference distance of 60 mm after the maskfilter 303 was removed.

[0109] While a change of the dimensions of film 101 was 50 to 80 μmrelative to a reference distance of 60 mm in the first embodimentwherein the mask films 103 are formed on the both sides of the film 101by a laminating process, effects of residual strain like laminatingprocess is almost never found and the dimensional accuracy issignificantly improved in this embodiment wherein the mask films 303 areformed by applying and drying a resin varnish 304.

[0110] Then, as shown in FIG. 3F, an interconnection layer 309 supportedby a supporting substrate 308 and a copper foil 310 are overlaidrespectively on the both sides of the film 301. In overlaying procedure,both of the them are overlaid in such a way the interconnection layer309 is at least located directly above the through hole 306 filled withconductive paste 307. And, both of them overlaid are heated andpressurized. Heating and pressurizing is carried out, for example, byusing vacuum press.

[0111] The adhesive layer 302 is fluidized and the interconnection layer309 is buried into the adhesive layer 302 by these heating andpressurizing as shown in FIG. 3G. The conductive paste 307 within thethrough holes 306 is compressed by that the interconnection layer 309 isburied into the adhesive layer 302 like this, and therefore a resiningredient in the conductive paste 307 flows out into the adhesive layer302 and a conductive element in the conductive paste 307 is closelypacked. Thus, the interconnection layer 309 and the copper foil 310which are located on the opposite sides of the film 301 are electricallyinterconnected through the intermediary of conductive paste 307.Hereafter, the adhesive layer 302 and the conductive paste 307 arecured.

[0112] Then, as shown in FIG. 3H, the copper foil 310 is patterned intodesired interconnection form using photolithography. In this embodiment,a foil in 9 μm thickness is utilized for the copper foil 310.

[0113] At the last step, the supporting substrate 308 is removed leavingthe interconnection layer 309 buried into the adhesive layer 302 asshown in FIG. 3I. Thus, a circuit board having the interconnections onthe both sides is completed. In this embodiment, aluminum foil is usedfor the supporting substrate 308 and copper foil (patterned) in 9 μmthickness is utilized for the interconnection layer 309.

[0114] Removing the supporting substrate 308 is performed by dissolvingand removing the aluminum foil by selective etching between aluminum andcopper foils. Other than selective etching, there is also a method ofremoving the supporting substrate 308 by heating with releasing foamsheet provided between the supporting substrate 308 and theinterconnection layer 309. In this method, the supporting substrate 308is not limited to the aluminum foil, and metal foil and metal plate ofmaterials such as copper, stainless and the like may be utilized.

[0115] Further, in this embodiment, a multilayer interconnection boardmay be formed by treating the product prepared by the procedures of FIG.3A to FIG. 3H as the interconnection layer 309 supported by thesupporting substrate 308 in FIG. 3F and by repeating the procedures ofFIG. 3F to FIG. 3H.

[0116] In accordance with this embodiment, fine through holes 306, forexample, such as the diameter of 50 μm may be formed because holes aredrilled by using a YAG solid-state laser radiating laser light with ashort wavelength not longer than that in the ultraviolet range. Further,since the mask films 303 are formed by applying and drying a resinvarnish 304, a change in dimension of the substrate in removing a maskfilm may be decreased compared with the case that mask films are formedby a laminating process. A higher density circuit board may be realizedfrom these reasons.

[0117] Fourth Preferable Embodiment

[0118]FIG. 4 are sectional views of the steps illustrating a process forthe manufacturing of a circuit board in the fourth preferred embodimentof the present invention.

[0119] At first, as shown in FIG. 4A, the removable mask films 402 areformed on the both sides of the porous insulating substrate the 401having compressibility. The porous insulating substrate 401 is, forexample, prepared as follows. That is, thermosetting resin isimpregnated into nonwoven fabric utilizing organic fiber. Thereby,porous insulating substrate 404 having voids inside and compressibilityis prepared. Then, thermosetting resin impregnated is brought to thehalf-cured condition. Materials described in the second embodiment maybe used as organic fiber and thermosetting resin.

[0120] In this embodiment, nonwoven fabric impregnated with epoxy resinwhich uses aromatic polyamide fiber is used as the porous insulatingsubstrate 401. And, a thickness of the porous insulating substrate 401is 120 mm.

[0121] The mask film 402 is formed by the method that the resin varnish403 is applied on the both sides of the porous insulating substrate 401and dried. The resin varnish 403 is composed of at least a resin to be aprincipal component of the mask film 402, a ultraviolet-absorbing agentand solvent. Cellulosic, for example, such as nitrocellulose, acethylcellulose, cellulose acetate, cellulose propionate, ethyl cellulose andthe like may be used for a resin to be a principal component of the maskfilm 402.

[0122] When the mask film 404 is formed as a principal component ofcellulosic, a ultraviolet-absorbing agent described in the firstembodiment is added in such a way that hole drilling may be performed byusing laser light with short wavelength of 400 nm or less in the rangeof wavelength not longer than that in the ultraviolet range. Further, asfor solvent composing of the resin varnish 404, it is selected such asolvent that dissolves the principal component composing of the maskfilm 403 and the ultraviolet-absorbing agent but does not erode theporous insulating substrate 401. Transformation of the foregoingmaterial mixed into varnish may be achieved by using in combinationadequately dispersion equipment using a media such as a ball mill, asand mill or the like, kneading equipment such as a pressure kneader, aplanetary mixer or the like and mixing equipment such as a disper or thelike. The concentration and the viscosity of the resin varnish 403 maybe determined corresponding to a application means.

[0123] The method for applying the resin varnish 403 on the porousinsulating substrate 401 is not limited, a publicly known method such aselectrostatic coating, dip coating, spray coating, roller coating,doctor blade method, gravure method, die coating and screen printing etcmay be adopted.

[0124] In the fourth embodiment, the resin varnish 403 is prepared bymixing and dispersing materials having the following compositions.

[0125] Ethyl cellulose (ETHOCEL STD100:Dow Chemical Company)

[0126] 100 parts by weight

[0127] Ultraviolet-absorbing agent (JF-79: Johoku Chemical Co., Ltd.) 1parts by weight

[0128] Toluene 100 parts by weight

[0129] Methanol 800 parts by weight

[0130] The mask films 402 are formed by applying a resin varnish 403 onthe both sides of the porous insulating substrate 401. The resin varnish403 is applied by die coating using a die nozzle 404. A thickness of themask films 402 is adjusted to 10 μm after it is formed in such a way andthen dried. Drying is performed at a temperature required for releasingthe solvent e.g. the order of 100° C. Futher, the ultraviolet-absorbingagent in this embodiment (JF-79: Johoku Chemical Co., Ltd.) is composedof 2-(2′-hydroxy-3′-tert-butyl-5′-methyl phenyl)5-chlorobenzotriazole tobe a principal component.

[0131] Through holes 405 are in turn formed in the porous insulatingsubstrate 401 provided with mask films 402 on both sides of the porousinsulating substrate as shown in FIG. 4B. Through holes 405 are formedby laser processing.

[0132] In this embodiment, a third harmonics YAG solid-state laserradiating laser light with a wavelength of 351 nm not longer than thatin the ultraviolet range is used to perform a fine hole drilling as wellas the first embodiment described above. Through holes 405 of thediameter of 100 μm are formed by using such a technique.

[0133] Then, as shown in FIG. 4C, conductive paste 406 is filled intothe through holes 405. In this embodiment, the conductive paste 406 isfilled by printing the conductive paste 406 from on the mask film 402 bya screen printing machine. In this time, a resin ingredient in theconductive paste 406 within the through holes 405 is drawn by evacuatingand absorbing through a porous sheet (not shown) such as Japanese paperand the like from the opposite side to the printed face under vacuum.The conductive paste 406 is increased in a percentage of a conductiveelement by this operation and further closely filled.

[0134] When such procedures are performed, the mask film 402 plays rolesas a printing mask and a protector against contamination of the surfaceof the porous insulating substrate 401.

[0135] Then, as shown in FIG. 4D, the mask films 402 are removed fromthe both sides of the film 401. In this procedure, since the throughholes 405 are as fine as a diameter of 100 μm, a phenomenon of “thepaste being taken away” described in the first embodiment seldom occurs.Further in this time, a change of the dimensions of the porousinsulating substrate 401 was within 10 μm relative to a referencedistance of 60 mm after the mask filter 402 was removed.

[0136] While a change of the dimensions of film 101 was 30 to 50 μmrelative to a reference distance of 60 mm in the second embodimentwherein the mask films 202 are formed on the both sides of the porousinsulating substrate 201 by a laminating process, effects of residualstrain like laminating process is almost never found and the dimensionalaccuracy is significantly improved in this embodiment wherein the maskfilms 402 are formed by applying and drying a resin varnish 403.

[0137] Then, as shown in FIG. 4E, copper foils 407 are overlaidrespectively on the both sides of the porous insulating substrate 401.And, both of them overlaid are heated and pressurized. Heating andpressurizing is carried out, for example, by using vacuum press.

[0138] The conductive paste 406 is compressed and therefore a resiningredient in the conductive paste 406 flows out into the porousinsulating substrate 401 by these heating and pressurizing, as shown inFIG. 4F, and a conductive element in the conductive paste 406 is closelypacked. Thus, the copper foils 407 which are located on the oppositesides of the porous insulating substrate 401 are electricallyinterconnected each other through the intermediary of conductive paste406. Hereafter, the porous insulating substrate 401 and the conductivepaste 406 are cured.

[0139] Then, as shown in FIG. 4G, the copper foil 407 is patterned intodesired interconnection form using photolithography. In this embodiment,a foil in 18 μm thickness is utilized for the copper foil 407. Therefoe,a circuit board having the interconnections on the both sides iscompleted.

[0140] Further, in this embodiment, a multilayer interconnection boardmay be formed by treating the product prepared by the procedures of FIG.4A to FIG. 4G as the copper foil on the one side in FIG. 4E and byrepeating the procedures of FIG. 4E to FIG. 4G.

[0141] Fifth Preferable Embodiment

[0142] When drilling by laser processing, wastes resulting from thataramid prepreg substrate and removable mask film are melted by heat oflaser and melting film shatter adhere on the surface of the removablefilm. When conductive paste is printed by squeezy under such acondition, said waste is readily involved into the through hole. As aresult of analyzing the defective hole in actual, interconnection layer107 removable film component was detected in the conductive paste withinvia hole contact portion. Even though drilling method is altered to theanother such a machining using a drill, the result does not change.

[0143] Therefore, a plurality of removable films are laminated in thisembodiment. And, the scraping described above is removed by removed onlya superficial removable film after hole drilling and dust deposited onthe surface of the mask film during procedures up to the procedure forforming the through hole is removed by removing a superficial removablefilm after hole drilling.

[0144] Hereinafter, a process for the manufacturing of the circuit boardin this embodiment will be described. By the way, since this embodimentis essentially similar to the second embodiment except a composition ofthe removable mask film, the composition which is same as or similar tothat of the second embodiment is designated with the similar symbol andthe description on the composition will be omitted. FIG. 5 is sectionalviews illustrating characteristic steps in a process for themanufacturing of this embodiment.

[0145] At first, as shown in FIG. 5A, laminated mask films 202′ formedwith releasing layer on one side are pasted to the both sides of theporous insulating substrate 201 by laminating process. A laminated maskfilm 202′ is composed of two removable mask films 202A, 202B overlaidwith adhesive layers interposed between two removable films.

[0146] An enlarged sectional view of portion “A” of FIG. 5A is shown inFIG. 5A-1. As illustrated in this view, a laminated mask film 202′ iscomposed by laminating mask film 202A, a releasing layer 501, anadhesive layer 502, a mask film 202B and a releasing layer 501 insuccession from the outer. A film of polyethylene naphthalate (PEN) andthe like having absorptivity upon laser light with wavelength of 400 nmor less in the range of wavelength not longer than that in theultraviolet range is used as mask films 202A, 202B.

[0147] Through holes 203 are then formed in the porous insulatingsubstrate 201 by laser processing as shown in FIG. 5B. In thisembodiment, a third harmonics YAG solid-state laser radiating laserlight with a wavelength of 351 nm being not longer than that in theultraviolet range is used to perform a fine hole drilling as well as thefirst embodiment described above.

[0148] Further, as shown in FIG. 5C, a mask film 202A positioned outerside among mask films 202A, 202B composing a laminated mask film 202′ isremoved.

[0149] Then, as shown in FIG. 5D, conductive paste 204 is filled intothe through holes 203. With filling method, for example, the conductivepaste 204 is filled by printing directly from on the porous insulatingsubstrate 201 by using a screen printing machine.

[0150] Then, as shown in FIG. 5E, mask film 202B left is removed fromporous insulating substrate 201. Thereby, porous insulating substrate201 filled with conductive paste 204 is obtained.

[0151] Since the following manufacturing procedure is similar to thesecond embodiment described taken in connection with FIG. 2E to FIG. 2G,description on these procedure will be omitted.

[0152] In this embodiment, a laminated mask film 202′ comprising aplurality of layers of mask films 202A, 202B is laminated at least onthe one surface of porous insulating substrate 201 of the side filledwith conductive paste 204, and the superficial mask film 202A is removedafter drilling the through holes 203. Thereby, the through hole 203 isprotected from the waste generated during drilling the through hole 203contaminating into the conductive paste 204. As a matter of course, alaminated mask films 202′ provided on both sides of porous insulatingsubstrate 201 like this embodiment allow to protect the wastecontamination more effectively.

[0153] Further, in this embodiment, after the mask film 202′ islaminated on both sides of the porous insulating substrate 201respectively, the through hole 203 is drilled and further thesuperficial mask films 202 a are removed from the both sides of theporous insulating substrate 201 respectively. Alternately, in thisstage, removing the superficial removable mask film 202A may be limitedto the side filled with conductive paste 204 and the superficialremovable mask film 202A of the other side may also be left withoutremoving.

[0154] Or, the laminated mask film 202′ may laminated on the film sidefilled with conductive paste 204 and a single layer of removable maskfilm 202 may be laminated on the other side.

[0155] In this connection, it is needless to say that a process formanufacturing using laminated mask film 202′ of this embodiment may beapplied, in addition to this embodiment, for the first, the third andthe fourth embodiments as well. With the applications for anotherembodiment, a method of laminating a laminated mask film which ispreviously laminated on a substrate or a method of laminating a singlelayer of mask film on a substrate and thereafter bonding another maskfilm to the substrate with single mask film in order by the applicationand the like are be able to be considered.

[0156] Further, though a laminated mask film 202′ laminated with tworemovable mask film 202A, 202B is used in this embodiment, a laminatedmask film laminated with three or more removable mask films may be usedas well. For example, a laminated mask film laminated with threeremovable films through the medium of the adhesive is laminated on theporous insulating substrate 201. Thereafter, the through holes 203 areformed and after the most outer removable film is removed by one sheet,the conductive paste 204 is filled. And then, residual two sheet of theremovable film mask are removed in order after the paste is filled.Thus, remaining amount (protruding amount) of conductive paste 204filled into the through hole 203 and left in a configuration protrudingthrough the surface of the porous insulating substrate 201 increases.When copper foil is laminated and pressurized under this condition, adegree of compression of the conductive paste 204 is increased.

[0157] Increase of a degree of compression of the conductive paste 204allow to cause the connective resistance small and stable. For example,a laminated film laminated with two removable mask films in a thicknessof 9 μm and similarly a laminated film laminated with one removable maskfilm in a thickness of 18 μm after the removal of the most outerremovable mask are compared with each other.

[0158] Generally, when removable mask film is removed after theconductive paste is filled into the through hole, a part of theconductive paste within the through hole is take away according to“phenomenon of the paste being taken away” as a result of adhesion ofthe paste to the inner wall of the removable mask film hole. The amountof the paste taken away significantly increases as the contact area ofthe paste with the inner wall of the hole in the removable mask filmincreases.

[0159] Therefore, even when the overall thickness of removable maskfilms are equal to each other, when the removable mask film is removedseparately two or more times through having a laminated structure as twolayers, the overall amount of the paste taken away together with theremovable mask film may decrease.

[0160] Of course, since the numbers of processing and the manufacturingcost increase as the numbers of the layer in the laminated mask filmincreases, it is better to determine the optimal numbers of the layerlaminated considering the quality, the manufacturing cost and themanufacturing schedule managing.

[0161] A circuit board having the interconnections on the both sides ofthe board was manufactured according the procedures described in thisembodiment. And, as a reference, A circuit board having theinterconnections on the both sides of the board was obtained using asingle layer of removable mask film according the procedures.

[0162] A prototyping conditions are as follows. A prepreg substrateprepared by impregnating aramid nonwoven fabric with epoxy resin is usedas the porous insulating substrate 201. As removable mask film 202′, inthis embodiment, the laminated mask film 202′ laminated with tworemovable mask film 202A, 202B comprising PEN film in thickness of 16 mformed with a releasing layer on the one side through the medium of theadhesive layer 502 was used.

[0163] In the reference, the single layer of removable mask filmcomprising a single PEN film in thickness of 16 μm formed with areleasing layer on the one side was used. The through hole is formed indiameter of 200 μm by laser processing. As the conductive paste, copperpaste was used.

[0164] A circuit board in which a pitch of the drilled hole of thethrough hole 203 was varied in the four kinds of 0.4 mm, 0.8 mm, 1.2 mm,and 1.6 mm was manufactured.

[0165] With respect to the evaluation, the initial resistance value inconnecting 500 via holes contact portions in series (sum of theresistance values of 500 via holes contact portions and lines) aremeasured and evaluated by the values and variations.

[0166] Measurements are shown in FIG. 6. In the case that a single layerof the removable mask film indicated by the dotted line is (referenceembodiment case), the resistance value increases and the variations ofthe resistance is large as a pitch of the drilled via hole of thethrough hole becomes narrow. On the other hand, in the case that theremovable mask film 202′ is laminated on the one side shown by solidline (the present embodiment), a good results was obtained such that theresistance is constant independently the pitch of the drilled hole ofthe through hole 203 and the variations of the resistance is small.

[0167] In this embodiment, scraping generated in hole drilling and dustdeposited during a processing may be protected from contaminatingtogether with the paste in filling conductive paste 205 into the throughhole 203. Therefore, a circuit board having via hole contact of lowinitial resistance and high interconnection reliability may be obtained.Furthermore, since at least one removable mask film 202B is left on theside of the substrate when a superficial removable film 202A is removed,the removable mask film 202B functions as a printing mask in filling theconductive paste 205 and therefore a surface of the substrate is notcontaminated with the conductive paste 205.

[0168] Sixth Preferable Embodiment

[0169] A process for the manufacturing of the circuit board in thisembodiment will be described. By the way, since this embodiment isbasically similar to the second embodiment except a composition of theremovable mask film, the composition which is same as or similar to thatof the second embodiment is designated with the similar symbol and thedescription on the composition will be omitted. FIGS. 7A to 7E and 8A to8C are sectional views illustrating each step of a process for themanufacturing in this embodiment.

[0170] At first, as shown in FIG. 7A, the removable mask films 202 isformed on only the one side of the porous insulating the substrate 201having compressibility.

[0171] The porous insulating substrate 201 similar to that of the secondembodiment is used. That is, nonwoven fabric impregnated with epoxyresin which uses aromatic polyamide fiber is used as the porousinsulating substrate 201. A thickness of the porous insulating substrate201 is 120 μm.

[0172] The mask film 202 similar to that of the second embodiment isused. That is, a variety of film including, as an embodiment,polyethylene naphthalate (PEN) absorbing laser light with wavelength of400 nm or less not longer than that in the ultraviolet range is used asmask films. The mask film 202 is provided with a silicon base releasinglayer as required. And, a method of pasting the mask film 202 to theporous insulating substrate 201 is similar to the second embodiment.

[0173] Then, as shown in FIG. 7B, a supporting substrate 702 includingan interconnection layer 701 is overlaid on the other side (not formedwith mask film 202) of the film 201. And, both of them overlaid isheated and pressurized. Heating and pressurizing is carried out, forexample, by using vacuum press.

[0174] Base-blind holes 703 are in turn formed in the porous insulatingsubstrate 201 provided with a mask film 202 by laser processing as shownin FIG. 7C. In this processing, base-blind holes 703 are formed by usinga third harmonics YAG solid-state laser radiating laser light with awavelength of 351 nm as well as the second embodiment described above.The base-blind hole 703 is formed inwardly from the face formed withmask film 202 to the inside of porous insulating substrate 201 in depthsubstantially equivalent to a thickness of porous insulating substrate202. In this procedure, the base-blind hole 703 is aligned with theinterconnection layer 701 and the formed. Thereby, base-blind hole 703is formed in a configuration that the base-blind hole goes through themask film 202 and porous substrate 201, and interconnection layer 701 isexposed at the base.

[0175] The porous insulating substrate 201 has a characteristic toabsorb laser light with a wavelength in the ultraviolet range and a maskfilm 202 has also a characteristic to absorb laser light with awavelength of 400 nm or less as described above. Therefore, base-blindholes 703 may be formed by using a third harmonics YAG solid-state laserlight with a wavelength of 351 nm. In this embodiment, base-blind holes703 holes of the diameter of 100 μm are formed by using a laser light aswell as the second embodiment.

[0176] Then, as shown in FIG. 7D, conductive paste 204 is filled intothe base-blind holes 703. In this embodiment, the conductive paste 204is printed directly from on the mask film 202 by a screen printingmachine.

[0177] When such procedures are performed, the mask film 202 plays rolesas a printing mask and a protector against contamination of the surfaceof the porous insulating substrate 201.

[0178] Then, as shown in FIG. 7E, the mask films 202 and the supportingsubstrate 702 are removed from the porous insulating substrate 201. Inthis procedure, since a diameter of the base-blind holes 703 are 100 μm,a phenomenon of “the paste being taken away” described in the firstembodiment seldom occurs.

[0179] Then, as shown in FIG. 8A, copper foil 704 is overlaid on the onesides (side with the mask film removed) of the porous insulatingsubstrate 201. And, both of them overlaid are heated and pressurized.Heating and pressurizing is carried out, for example, by using vacuumpress.

[0180] The conductive paste 204 within the base-blind holes 703 iscompressed and therefore a resin ingredient in the conductive paste 204flows out into the porous insulating substrate 201 by these heating andpressurizing, as shown in FIG. 8B. Thus, a conductive element in theconductive paste 204 is closely packed and the copper foils 704 and theinterconnection layer 701 which are located on the opposite sides of theporous insulating substrate 201 are electrically interconnected eachother through the intermediary of conductive paste 204. Hereafter, theporous insulating substrate 201 and the conductive paste 204 are cured.

[0181] Then, as shown in FIG. 8C, the copper foil 704 is patterned intodesired interconnection form using photolithography. Thus, a circuitboard having the interconnections on the both sides is completed.

[0182] Removing the supporting substrate 702 is performed by dissolvingand removing the aluminum foil by selective etching between aluminum andcopper foils. Other than selective etching, there is also a method ofremoving the supporting substrate 702 by heating with releasing foamsheet provided between the supporting substrate 702 and theinterconnection layer 701. In this method, the supporting substrate 702is not limited to the aluminum foil, and metal foil and metal plate ofmaterials such as copper, stainless and the like may be utilized.

[0183] Further, in this embodiment, a multilayer interconnection boardmay be formed by treating the product prepared by the procedures of fromFIG. 7A to FIG. 7E and from FIG. 8A to FIG. 8C as the supportingsubstrate 702 with the interconnection layer in FIG. 7B and by repeatingthe procedures of from FIG. 7B to FIG. 7E and from FIG. 8A to FIG. 8C.

[0184] In this embodiment, because the base-blind hole 703 is formedagainst porous insulating substrate 201 laminated an located on theinterconnection layer 701 toward interconnection layer 701, alignmentbetween the interconnection layer 701 and the base-blind hole 703,therefore, alignment between the interconnection layer 701 andconductive paste 704 has a high degree of precision.

[0185] Then, optical absorptivity upon laser light of wavelength notlonger than that in the ultraviolet range, which is specified inselecting a removable film in each embodiment, will be described. FIGS.9 to 12 are graphs indicating optical absorptivity of each specimen uponlaser light with a wavelength of 351 nm radiated by a third harmonicsYAG solid-state laser: FIG. 9 shows optical absorptivity of polyethylenenaphthalate (PEN); FIG. 10 shows optical absorptivity of polyethyleneterephthalate (PET); FIG. 11 shows optical absorptivity of species inwhich a ultraviolet-absorbing agent (JF-77: Johoku Chemical Co., Ltd.:Addition rate 1 part by weight) is added to ethyl cellulose; and FIG. 12shows optical absorptivity of a single substance of ethyl cellulose. Inthese drawings, a lateral axis and a vertical axis indicate wavelengthand optical transmittance, respectively.

[0186] As it is apparent from these Figures, polyethylene terephthalate(PET) has optical transmittance of 80% and does not show sufficientoptical absorptivity. On the other hand, polyethylene naphthalate (PEN)shows high optical absorptivity. And, a single substance of ethylcellulose does not show optical absorptivity. On the other hand, ethylcellulose added with a ultraviolet-absorbing agent shows good opticalabsorptivity to obtain sufficiently the effects of the presentinvention.

[0187] Further, as it is apparent from these Figures, removable maskfilm has preferably optical transmittance of at most 40% or less uponlaser light in a range of wavelength not longer than that in theultraviolet range for exerting effects of the present invention.

[0188] As described above, in each embodiment of the present invention,removable mask film having a characteristic to absorb laser light ofwavelength not longer than that in the ultraviolet range is used. Thus,hole drilling may be performed by using laser light having a relativelyshort wavelength not longer than that in the ultraviolet range in whichlaser beam is easy to focus more. Therefore, fine hole drilling may beperformed and a high density circuit board having a fine via hole may berealized.

[0189] Further, since the removable mask films are formed on at leastthe one side of the substrate by applying and drying a resin varnish, achange in dimension of the substrate in removing a removable film may bedecreased compared with the case that removable mask films are formed onthe substrate by a laminating process, and thus a high density circuitboard with a high dimensional accuracy may be realized.

[0190] In each embodiment described above, hole drilling is performed byusing YAG solid-state laser but the type of laser is not limited tousing YAG solid-state laser. Any type of laser which has a wavelengthnot longer than that in the ultraviolet range, for example, excimerlaser or others may be used for drilling.

[0191] Though, in each embodiment described above, it has been describedabout an embodiment of applying for a process for manufacturing of thecircuit board with interstitial via hole contact performed by conductivepaste, the removable film of the present invention may be applied for aprocess for manufacturing of another circuit board.

[0192] Though the mask films are formed on both sides of the substratein each embodiment described above, they may be formed on only the oneside of the substrate.

[0193] While the present invention has been described in detail inconnection with the most preferred embodiment, various modifications ofcombination and arrangement of the components in the preferredembodiments may be made without departing from the spirit and scope ofthe present invention claimed hereinafter.

What is claimed is:
 1. A removable film having: removability whereinsaid removability enables said film to be removed from said substrateafter said film is drilled by laser light along with said substrate withsaid film provided on a surface of said substrate; and opticalabsorptivity absorbing laser light in a range of wavelength not longerthan that in the ultraviolet range.
 2. The removable film as set forthin claim 1, wherein said wavelength not longer than that in theultraviolet range is a wavelength of 400 nm or less.
 3. The removablefilm as set forth in claim 1, wherein said optical absorptivity isspecified by a condition of optical transmittance of 40% or less.
 4. Theremovable film as set forth in claim 1, wherein said removable film isused as a protective mask when a conductive element is filled into adrilled hole formed in said substrate by said drilling.
 5. The removablefilm as set forth in claim 1, wherein said removable film contains aultraviolet-absorbing agent.
 6. The removable film as set forth in claim1, wherein said removable film is predominantly composed of cellulosic.7. The removable film as set forth in claim 6, wherein said cellulosicare nitrocellulose, acethyl cellulose, cellulose acetate, cellulosepropionate and ethyl cellulose.
 8. The removable film as set forth inclaim 1, wherein said removable film is predominantly composed ofpolyethylene naphthalate (PEN), polyamide or polyimide.
 9. A substratewith film comprising: a substrate drilled on a surface of said substrateby laser light; and a removable film provided on a surface of asubstrate and removed from said substrate after hole drilling by saidlaser light; wherein said removable film have optical absorptivityabsorbing laser light in a range of wavelength not longer than that inthe ultraviolet range.
 10. The substrate with film as set forth in claim9, wherein said wavelength not longer than that in the ultraviolet rangeis a wavelength of 400 nm or less.
 11. The substrate with film as setforth in claim 9, wherein said optical absorptivity is specified by acondition of optical transmittance of 40% or less.
 12. The substratewith film as set forth in claim 9, wherein said substrate is a filmsubstrate having optical absorptivity absorbing laser light ofwavelength not longer than that in the ultraviolet range and adhesion.13. The substrate with film as set forth in claim 9, wherein saidsubstrate is a porous substrate having optical absorptivity absorbinglaser light of wavelength not longer than that in the ultraviolet rangeand compressibility.
 14. The substrate with film as set forth in claim9, wherein said removable film contain an ultraviolet-absorbing agent.15. The substrate with film as set forth in claim 9, wherein saidremovable film are predominantly composed of cellulosic.
 16. Thesubstrate with film as set forth in claim 15, wherein said cellulosicare nitrocellulose, acethyl cellulose, cellulose acetate, cellulosepropionate and ethyl cellulose.
 17. The substrate with film as set forthin claim 9, wherein said removable film are predominantly composed ofpolyethylene naphthalate (PEN), polyamide or polyimide.
 18. Thesubstrate with a film as set forth in claim 9, wherein said a pluralityof removable film are laminated and provided on said substrate.
 19. Aprocess for forming a removable film having removability wherein saidremovability enables said film to be removed from said substrate aftersaid film is drilled by laser light along with said substrate on thesurface of said substrate and optical absorptivity absorbing laser lightin a range of wavelength not longer than that in the ultraviolet rangeby applying and drying a resin varnish, which exerts removability afterdrying and has optical absorptivity upon laser light in a range ofwavelength not longer than that in the ultraviolet range, on the surfaceof said substrate.
 20. The process for forming a removable film as setforth in claim 19, wherein said resin varnish contains a solvent noteroding said substrate.
 21. A process for the manufacturing of a circuitboard comprising the steps of: a procedure for providing a removablefilm absorbing laser light in a range of wavelength not longer than thatin the ultraviolet range on a surface of a insulating substrateabsorbing laser light in a range of wavelength not longer than that inthe ultraviolet range; a procedure for forming a hole from a sideprovided with the film in said insulating substrate by using laser lighthaving a wavelength not longer than that in the ultraviolet range; aprocedure for filling a conductive element into said drilled hole; aprocedure for removing said removable mask film from said insulatingsubstrate; and a procedure for interconnecting electrically between saidconductive element and a conductive element pattern or between saidconductive element and metal foil by overlaying said conductive elementpattern or said metal foil supported by a supporting element on thesurface of said insulating substrate and by pressurizing and heatingsaid conductive element pattern or metal foil overlaid.
 22. A processfor the manufacturing of a circuit board as set forth in claim 21,wherein a porous insulating substrate having compressibility is used assaid insulating substrate.
 23. A process for the manufacturing of acircuit board as set forth in claim 21, wherein a film substrate havinga adhesive layer on the surface thereof is used as said insulatingsubstrate.
 24. A process for the manufacturing of a circuit board as setforth in claim 21, wherein a procedure for providing said removable maskfilm on said insulating substrate includes a procedure for applyingresin varnish on the surface of said insulating substrate and aprocedure for drying said resin varnish applied.
 25. A process for themanufacturing of a circuit board as set forth in claim 21, wherein aplurality of removable films are laminated and said laminated film isused as said removable film in said procedure for providing saidremovable mask film on said insulating substrate, the process furthercomprising a procedure for removing selectively only a superficialremovable film after a procedure for drilling said insulating substrate,and wherein the procedure for filling a conductive element into saiddrilled hole is performed after removing the superficial removable film.26. A process for the manufacturing of a circuit board as set forth inclaim 25, wherein at least two said removable mask film are left on saidinsulating substrate in the procedure for removing the superficialremovable mask film.